1. Uses of solar hard X-rays Basics of observations Hard X-rays at flare onset The event of April 18, 2001 Conclusions Yohkoh 10th Jan. 21, 2002Hugh Hudson, SPRC/UCB

2. Hard X-ray basics Hard X-rays come from bremsstrahlung Bremsstrahlung is inefficient, and electrons >20 keV dominate flare energetics Flare hard X-rays normally follow the Neupert effect Footpoints (often double) are prominent

3. Basics II Spectra look like power laws (may be broken) Impulsive-phase hard X-rays follow the Soft-hard-soft pattern Protons > 10 MeV now appear to resemble electrons > 20 keV energetically Also see “superhot” hard X-rays

4. Hot “Superhot” Broken power law Gamma rays “Broken up” Pi decay ? 1 keV 1 MeV1 GeV E f(E)

5. Another Neupert plot

6. A non-Neupert event

7. Conclusion Energy release in the impulsive phase coincides temporally with a distorted electron distribution function.

8. Neupert Effect - II Correlation plot for hard X-ray and soft X- ray peak fluxes, done for "slow LDEs"

9. A typical slow LDE, the “candle flame” flare (21-Feb-92)

10. Feb. 21 has a long “impulsive” phase - the Neupert effect

11. Conclusion The “impulsive phase” non-thermal effects occur in flares of all sizes and time scales.

12. Conclusion Soft-hard-soft spectral evolution is something fundamental that is modelable, but which has not yet been modeled.

13. September 24, 2001 HXT observed a rapidly-variable source with a soft spectrum at the onset phase

14. Conclusion There are strong suggestions that the basic physics of pre-flare excitation is not “heating,” but rather particle acceleration

15. April 18, 2001

16. Summary of uses I Footpoint connectivity Impulsive-phase energetics Coronal trapping Filament activation “Firm” X-rays

17. Summary of uses II Type I Type II Type III Type IV

18. Hot “Superhot” Broken power law Gamma rays “Broken up” Pi decay ? 1 keV 1 MeV1 GeV E f(E)

19. Reflecting hard X-ray telescope Zero background Infinite dynamic range Photometry limited by counting statistics Good spectral domain (to tens of keV) All technologies seem to be available